Conventionally, as an impeller used for a centrifugal rotating machine such as a centrifugal compressor, there has been known a covered impeller (closed impeller) including a disc attached to a rotating shaft, a cover installed with a space being provided with respect to the disc, and a plurality of blades for connecting the cover to the disc. For this impeller, a portion surrounded by the side surfaces of blades, the cover, and the disc constitutes a passage for compressing air.
As a method for manufacturing this impeller, there has been known a method in which the disc and the cover, which are produced separately, are joined to each other. In this method, the blades are often formed integrally with either of the disc and the cover. As a method for joining the disc and the cover to each other, welding or brazing is selected. In the joining by welding, a welding torch must be inserted into the passage, and it has been pointed out that if the passage is narrow, it is difficult to insert the welding torch into the passage, and therefore welding defects are liable to occur. In contrast, in the joining by brazing, for example, when the cover with blade is joined to the disc, a brazing filler metal is placed on the joint surface, and the cover and the disc are inserted into a heating furnace in a lapped state to join them, so that even if the passage is narrow, the joining work can be performed easily.
Patent Document 1 proposes a method for joining the disc to the cover by brazing. In this method, the cover is placed so that the blade mounting surface faces up, and brazing is performed in the state in which the blade and disc are placed on the cover. According to the method proposed by Patent Document 1, insufficient supply of brazing filler metal to a portion that is a joint portion of the blade to the cover and the disc, on the inner periphery side in the radial direction of impeller, on which portion the rotational stresses at the impeller operation time concentrate, is prevented.